For obtaining high-purity methylal, there is a process using a molecular sieve made of synthetic zeolite or the like. This process, however, needs a very large amount of synthetic zeolite when a large amount of methylal is purified on an industrial scale. Moreover, in the process, since the adsorptivity of synthetic zeolite is deteriorated with the proceeding of the adsorption, regeneration of synthetic zeolite is necessary at a certain timing. The regeneration is conducted by heating the used synthetic zeolite at 250-300.degree. C. and passing nitrogen therethrough to remove the adsorbed water. Thus, much labor and energy are required and the process using synthetic zeolite is not convenient.
JP-A-56-147739 discloses a process for purification of methylal by adding water to crude methylal containing methanol and subjecting the mixture to distillation to separate and remove the methanol. According to the Example of the literature, however, it is prerequisite for the process that the content of methanol as impurity in crude methylal is high, and the methanol content is certainly reduced from 6.9% by weight to 1.1% by weight. Meanwhile, water also present in crude methylal forms an azeotropic mixture with methylal or methanol; therefore, it is impossible to obtain methylal containing water in an amount as small as 0.7% by weight or less.
It is well known that a polyacetal resin can be obtained by copolymerizing trioxane with ethylene oxide, dioxolan or butanediolformal. For example, in JP-B-36-14460 is disclosed copolymerization between trioxane and ethylene oxide or between trioxane and a cyclic formal (e.g. dioxolan, 1,4-butanediolformal). It is also well known that in this copolymerization, methylal is used as a molecular weight modifier. Since methylal contains impurities such as water, methanol and the like and these impurities invite molecular weight reduction and quality deterioration of produced polyacetal resin, it is desired to develop a process for purifying methylal to a higher purity. Since methylal forms an azeotropic mixture with water and methanol [the azeotropic point between methylal and water: 42.3.degree. C. (water content: 1.4% by weight), the azeotropic point between methylal and methanol: 42.3.degree. C. (methanol content: 7.8% by weight)], it is impossible to separate methylal from the mixture by simple distillation.
It has been a problem to be solved in the art to simply separate and remove water and methanol as impurities present in methylal on an industrial scale to obtain high-purity methylal.